DYNAMICS. This branch of mecha nics relates to the action of forces that give motion to solid bodies ; which forces are calculated, both by their active pow ers, and by the proportion of time in which those powers become efficient. Our readers cannot fail to perceive, that the complete analysis of all appertaining to this subject would occupy many formi dable volumes ; while the generality of those who. have absolute occasion to ac quire a complete knowledge of dynamics, would be led to consult the various ela borate publications that have been pub lished, for the edification of such as pos sess that disposition. We must, necessa rily, study simplicity, so far as our sub ject may admit, and endeavour to bring the most prominent matters into a mode rate compass.
Each body is considered as a mass of atoms divisible ad infinitum; the bulk or substance of a mass we consider as hav ing density, which relates directly to the quantity of the matter, and inversely re fers to the magnitude. We are also corn pelled to consider, that, as the generality of bodies are more or less porous, their , quantity of matter is not in every instance I found to correspond with the bulk they exhibit. Thus we find that a pound of gold and a pound of lead, though appal rently solid, give very different weight o, within the same bulk. The former, is said to be more dense, while the latter is aid to be more rare ; density and rarity 'teeing opposite qualities. Therefore, if we could always ascertain the number of atoms, or of minute parts, contained in a mass, we should be able to appreciate the density ; because, under the certainty that, in proportion as more or less atoms are comprised Within a given space, so must the mass be more or less dense ; i. e. heavier or lighter.
In computing density we therefore take the; rectangle of the mass, and, ascertain ing the dimensions, discover by its weight how many atoms, or particles, it contains. hence is derived our table of specific gra vities, or the comparative weights of va rious bodies of unequal densities. (See 11 YU ItOSTAT16.) From this it will be seen that bodies of similar substance and form, but differing in bulk, are to each other in proportion to the mag nitude of their respective masses ; while, on the other hand, bodies of similar form, and equal in bulk, are, to each other, in proportion to their respective densities.
, Forces are considered according to the quantities of motion they are capable of producing ; but, as we cannot measure those forces, we are under the necessity of ascertaining the power by means of the effect. Thus we can correctly ascer tain the force of gunpowder by the ef fect produced by the shot ; or we can fully explain the force with which the spring of a watch acts, by finding what re sistance it is capable of overcoming ; or we may compute the strength of a horse, by witnessing the weight he can draw. But it must be obvious we could not discover, per se, either the strength of the gunpowder, the elastic powers of the spring, or the muscular vigour of the animal.
Force and velocity are, in fact, synony mous terms ; for the impetus given to the shot fired from a cannon, estimated by the bulk of the shot, and the distance to which it may be projected, or the im pression it may make on an opposing ob ject, completely supply the result of our research ; remarking, that, this being a diminishing force, its action will be strongest at the moment of expulsion, and gradually less as it recedes from the origin of motion, until it finally acquires a state of rest. The spring in a measure partakes of the same diminishing tenden cy ; but as it may be held in equilibrio at any period of its exertion, it cannot be classed with the former, though, rigidly speaking, it is assuredly a diminishing& force ; for, as we see in clocks and watch es, springs will in due time arrive at a state of rest, or inaction. The animal power is subject to so many anomalies, that it is next to impossible to treat of it with any strict adherence to calculation ; because, in so doing, we are compelled to banish what we know to be the effects of labour, and to consider the power as always equal, and always maintaining the same physical ability. Here, indeed, we find theorists generally proceeding upon a wrong basis ; and, of course, rarely correct in their conclusions. We find them estimating the powers of horses, &c. as though their limhs'had no flexibili ty, their muscles no relaxation, and as if their shoulders were insensible to pres sure ; in fact, they generally consider the animal as a fulcrum of wood, iron, &e. The absurdity of such a calculation must be obvious.